1. Field of the Invention
The present invention relates to a projection exposure apparatus for illuminating a first object with light to reduction-project a pattern on the first object thus illuminated onto a substrate or the like as a second object. More particularly, the invention relates to a projection exposure apparatus suitable for projecting a circuit pattern formed on a reticle (mask) as the first object onto a substrate (wafer) as the second object to effect exposure thereon.
2. Related Background Art
As patterns for integrated circuits become finer, demands for performance of the projection exposure apparatus used in printing on a wafer are becoming increasingly tougher these days.
Under such circumstances, a projection optical system is required to have a higher resolving power, flatness of image plane, less distortion, etc. Because of those, attempt has been made to reduce the distortion by shortening an exposure wavelength .lambda., increasing the numerical aperture NA of the projection optical system, or decreasing the curvature of field. Some examples of such attempts are those described in U.S. Pat. No. 5,260,832, Japanese Laid-open Patent Application No. 5-173065, etc.
Also, Japanese Laid-open Patent Applications No. 59-144127 and No. 62-35620 describe methods for adjusting only a magnification error. The former describes a technique that a curved film, for example a pellicle, which is very thin and which does not affect imaging performance, is set in an optical path. The latter describes a technique that a rotationally symmetric plano-convex lens or a combination of rotationally symmetric plano-convex and plano-concave lenses is moved along the optical axis to isotropically adjust the overall magnification on the wafer surface.
The high-performance projection optical systems as disclosed in U.S. Pat. No. 5,260,832 and Japanese Laid-open Patent Application No. 5-173065, however, include a lot of constituent lenses, i.e., 15 to 24 lenses. Particularly, in the case of high-resolution projection optical systems with numerical aperture NA being at least 0.4, the number of constituent lenses is very large, i.e., 20 or more. Thus, as the demand performance becomes higher, the projection optical systems are further increasing the number of constituent lenses and are becoming very complicated in structure. Therefore, in order to actually produce these projection optical systems, to mount them on projection exposure apparatus, keeping aberrations such as the curvature of field, the astigmatism, the distortion, etc. within ranges as designed, and then to obtain high performance, the accuracy of individual lens components and the accuracy of assembling must be controlled very strictly, which would raise problems of poor yield, very long production period, or failing to deliver sufficient performance, etc.
Further, the method for correcting the magnification error as described in Japanese Laid-open Patent Application No. 59-144127 includes a step of curving a very thin film or the like not affecting the imaging performance of the optical system so as to correct the magnification error by the prism effect thereof, but it cannot make fine adjustment for a correction amount or a correction direction of an asymmetric magnification error component with directionality remaining in the projection optical system. In addition, because it employs the thin film, the film can be two-dimensionally held as bonded on a metal frame or the like for long and narrow exposure areas as in the mirror projection method, but it is very difficult for such a thin film to be three-dimensionally held and to reveal good reproducibility for rectangular or square exposure areas. If glass etc. is used instead of the thin film for holding the shape, it is also difficult to form a thin and uniform film without affecting the imaging performance. Further, there are serious problems, e.g., durability of the film etc. including breakage accident due to heat absorption or the like of exposure light in actual use of such films etc., a change in optical performance with heat absorption of exposure light or with an environmental change, etc.
Further, Japanese Laid-open Patent Application No. 62-35620 discloses the technique that the magnification error is adjusted using a rotationally symmetric lens, but only moving the rotationally symmetric lens along the optical axis can adjust only the overall magnification on the wafer surface only on an isotropic basis and cannot adjust the asymmetric magnification error component with directionality remaining in the projection optical system.
Moreover, the methods for correcting the magnification error as disclosed in Japanese Laid-open Patent Applications No. 59-144127 and No. 62-35620 can basically correct only the magnification error, but they cannot correct the astigmatism etc. as off-axial aberrations. Further, it was also difficult for the methods to handle rotationally asymmetric magnification error components or distortion components locally remaining at random in the projection optical system.